It's time for my weekly post to this old thread. The grammar has grown enough to deserve more than one file, and is starting to change in new directions. For example, it's now Turing-complete, if you have a Parrot engine and a bit of spare time. Call it a primitive "demo version" of some of Perl 6's features. If nothing else, it will let you start training your fingers to 'concatenate' _$like _$this.
What's currently "supported": - most binary operators - arithmetic hyper-operators - chained comparisons - the ternary operator (both r- and l-value) - if/elsif/else (even "unless" and the feared "elsunless";) - void subroutines with declared parameter lists (only) - local variables (only -- i.e. "use UberStrict"). - arrays and scalars (only -- no hashes, closures, references) The fact that I've been able to whip this up in a couple thousand lines of code is a remarkable testament to Parrot's maturity, and to the wealth of tools available in Perl 5. In particular, without The Damian's Parse::RecDescent, Melvin Smith's IMCC, and Sarathy's Data::Dumper, it never would have been possible. Grammar fixes: - Fixed a bug with parameter initializers: "sub f($x = 23, $y)" now parses as "f(($x = 23), ($y))", not "f($x = (23, $y))" - Fixed a number of bugs sent in by Jonh Kingsley: - "sub f($a; $b, *@c)" (i.e. a sub with both optional and slurping parameters) now parses. - low-precedence 'and', 'or', 'xor' are now allowed within suitably-parenthesized expressions, such as "if (1 and 2)". Interestingly, you can now put a multi-dimensional array as the test of an "if". This is probably a feature that will go away at some point in the future. - "if (a && b)" is parsed correctly, not as "if (a & (&b))" - "f(2,3),4" reverts to "(f(2,3)),4" rather than "f((2,3),4)" through a bit of chicanery that allows "if" and other control structures to take parens around their first arguments. - "%a{somefunction($withargs)}" is legal. It used to see 'somefunction' and treat it as a bare hash key, then freak out about the open paren. - "last if $foo" and related cases where a guard was parsed as a label name. - empty programs and blocks - "stmt until expr" Changes: - Kingsley's changes: - handles comments - foreach decl(?) '(' expr ')' block - statement guards (e.g. "do_something() unless $foo") cannot be used with functions taking a closure as their last argument (e.g. "for", "elsunless"). This avoids a nasty ambiguity with things like this: if 1 { A } if foo { B } ==> guarded_stmt("if(1, {A})", if, "foo({B})") or ==> stmts("if(1, {A})", "if(foo(), {B})") - significant reorganization and tidying of files. The grammar now lives in its own module, and doesn't stomp on main's namespace with wild abandon. It's not a simple one-file install anymore, but it's getting big enough to maybe justify jumping through more hoops. - "Human-readable" output is gone, as it was more trouble than it was worth to maintain. The default now is to output the parse tree through Data::Dumper (there may be other tools to inspect Perl data structures in a more legible format). Next to do: - Function return values. - More informative error messages. They're currently... cryptic. - Having no built-in functions is kind of frustrating. Adding a few more things would be nice. - sub declarations. A standard function for turning a prototype into an argument context rule is needed eventually, and would eliminate the current nasty builtin hack in the compiler and parser. - rip out - Globals. - Handling of "closures" for if-blocks is completely bogus now -- they're always inlined, as there are no nested scopes. Once these are in, the current closure-handling will go away. (If you've read this far, I'm impressed. It took me several tries to get here myself) /s
grammar.tgz
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